A recent Tumblr change added an extra layer of security, breaking most of XKit features that deals with editing or creating posts.
Eight extensions are updated to work with the changes. You can wait for a while for XKit to update itself, or o get the updates immediately, you can go to XKit Control Panel > Other > Update All.
If you are still getting errors publishing posts or asks after updating every extension and upgrading to 7.3.0 or higher, please send me an ask. Thank you!